Application of short-wavelength lasers having a wavelength that falls within the deep ultraviolet range from 300 nm to 210 nm or vacuum ultraviolet range of 200 nm or shorter is expected to increase in various fields, including material processing, high-density optical recording, medical care, and sterilization, and as high-brightness white luminescent light sources. As laser sources having a wavelength that falls within these ranges, excimer lasers such as KrF (oscillation wavelength: 248 nm) and ArF (oscillation wavelength: 193 nm) are known. However, continuous wave oscillation is not allowed with these laser systems, and repetition frequency cannot be increased in pulse oscillation. Consequently, the energy per pulse increases, thus resulting in damage to optical components. Furthermore, since toxic fluoride gas is used, the laser systems require cumbersome maintenance, and consequently maintenance cost increases. In addition, these laser systems are large, and their laser beam quality is low. Due to those disadvantages, their application is limited, and that is why creation of downsized solid-state deep ultraviolet laser systems has been aspired.
Use of laser diodes has been studied to achieve downsized solid-state lasers. Deep-ultraviolet laser diodes using aluminum nitride (AlN), whose band gap is 6.4 eV, or mixed-crystal semiconductors consisting of aluminum nitride and gallium nitride (GaN), have been developed vigorously. However, they have problems that sufficient luminous efficiency cannot be obtained due to the problem of crystalline property and that the output is small (Non-patent Reference 1).
Meanwhile, as another method for achieving downsized solid-state laser systems, a solid-state laser is combined with a wavelength conversion element using a non-linear optical crystal to obtain coherent deep-ultraviolet light. Based on this combination, continuous wave oscillation is allowed in principle, and repetition frequency can also be increased in pulse oscillation. Furthermore, narrower bandwidth may also be allowed, and quality of mode in space is high.
The non-linear optical crystal is defined as a crystal having non-linear optical effect, namely the effect of non-linearity on the polarization response of materials. Specifically, the non-linear optical effect is defined as a phenomenon in which polarization response of a material, to which high-intensity light such as a laser beam is input, becomes disproportional to the electric field of the incident light, converting the wavelength of a part of the incident light. Second-harmonic generation for taking out light with a wavelength half that of an incident light using the second-order non-linear optical effect is best known as a method for converting laser wavelengths to shorter ones. According to this method, a Nd:YAG laser (wavelength: 1064 nm) can be converted into the one having the wavelength of 532 nm, which is then converted into the one with the wavelength of 266 nm by subjecting it to another wavelength conversion.
With this method, however, because of refractive index dispersion of the non-linear optical crystal to be performed wavelength conversion, the wavelength of the second harmonic within the crystal is not reduced precisely to the half of that of the incident light. Consequently, phase shift occurs between the second harmonic waves generated at various places within the crystal, which makes it difficult to obtain second harmonics having sufficient intensity. To solve this problem, phases are adjusted in general by using crystal orientation that allows the wavelength ratio of the incident light to that of the second harmonic to be precisely 2:1, utilizing the birefringence of the crystal.
However, in phase matching method using birefringence, it is impossible to make phase matching when it is exceeded the birefringence of the crystal. The quasi-phase matching method was proposed as a technique for matching the phases exceeding the limit of non-linear optical crystals (Patent Reference 1).
Quasi-phase matching can be achieved by periodically forming polarization reversal structures to a non-linear optical crystal. According to the quasi phase matching method, even if the non-linear optical crystal does not have appropriate birefringence at desired wavelength, the conversion efficiency can be improved by matching the phases of the fundamental wave and the second harmonic. Since the wavelength conversion based on quasi-phase matching does not use the birefringence of crystals, reduction in conversion efficiency resulting from different traveling directions of the fundamental wave and the second harmonic as well as degradation of beam quality can be prevented, which is an advantage of this method.
As wavelength conversion elements using the quasi-phase matching method, ferroelectric crystalline oxides such as LiNbO3 and LiTaO3 are known. Since the absorption edge of these ferroelectric crystalline oxides exists in proximity of the wavelength of 300 nm, the wavelengths usable in wavelength conversion elements using these crystals is limited to wavelengths of 300 nm or longer. Consequently, within the deep ultraviolet range from 300 nm to 210 nm, wavelength conversion using quasi-phase matching is not allowed. Furthermore, in practical wavelength conversion performed based on quasi-phase matching using LiNbO3 and LiTaO3, beam quality was found to have degraded within the wavelength range of 500 nm or shorter, 450 nm for example.
Against such technical background, a wavelength conversion element was proposed for generating deep ultraviolet rays with a wavelength of 300 nm or shorter, or vacuum ultraviolet rays with a wavelength of 200 nm or shorter, using a fluoride single crystal as the non-linear optical crystal, instead of LiNbO3 and LiTaO3 (Patent References 2 and 3).    Patent Reference 1: JP 2002-122898A (Claims, and a paragraph [0056])    Patent Reference 2: JP 2005-272219A (Claims)    Patent Reference 3: WO 2004/083497 (Claims)    Non-patent Reference 1: Y. Taniyasu, M. Kasu and T. Makimoto, Nature 441, 325 (2006)